F. Smend

Quasi-free Compton scattering and the polarizabilities of the neutron

Abstract:Differential cross-sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz photon tagging spectrometer at the Mainz MAMI accelerator together with the Mainz [48]cm ;SPMOslash; × [64]cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. The data cover photon energies ranging from [200]MeV to [400]MeV at θLABγ = 136.2°. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction p(γ,π+n). The “free” proton Compton scattering cross-sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross-section for free scattering from quasi-free data. Differential cross-sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron has been determined to be αn - βn = 9.8±3.6(stat)+2.1-1.1(syst)±2.2(model) in units of [10-4]fm3. In combination with the polarizability sum αn + βn = 15.2±0.5 deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, αn = 12.5±1.8(stat)+1.1-0.6(syst)±1.1(model) and βn = 2.7±1.8(stat)+0.6-1.1(syst)±1.1(model) are obtained. The backward spin polarizability of the neutron was determined to be γ(n)π = (58.6±4.0)×10-4fm4.

Measurement of γp→K0Σ+ at photon energies up to 1.55 GeV

Abstract The reaction γp → K 0 Σ + has been measured with the SAPHIR detector at the electron stretcher ring ELSA. The total cross section rises up to a peak value of 1.1 μ b at a photon energy of 1.4 GeV. The differential cross sections dσ/dΩ are consistent with being flat throughout the measured energy range. The first measurement of the Σ + polarization in photon induced reactions was obtained.

Low-background counting using Ge(Li) detectors with anti-muon-shields☆

Abstract Low-intensity photon counting experiments carried out in the energy range between 8 and 20 MeV using Ge(Li) detectors revealed a strong background due to cosmic muons. By using a lead shield to suppress photons and a five-sided 5 cm thick plastic shield to suppress muons, a background reduction of 99.5% was achieved. GEANT computer simulations are carried out to study properties of cosmic muon spectra in different Ge and Si detectors. A comparison of measured and simulated spectra leads to very good agreement. Background reduction by an annular NaI detector is studied in the anticoincidence and pair-spectrometer modes.

Pulse shape discrimination with a large NaI crystal

Abstract A large NaI Crystal (16 × 16 × 24 cm 3 ) is operated in a combined neutron and gamma-ray field in the energy range from 12 to 36 MeV. Pulse shape discrimination is employed to separate (n, charged particles) reaction products in NaI. Fluorescence efficiencies for various charged particles are studied.

Nuclear photoexcitation and delbrück scattering studied in the energy range 2–8 MeV

Elastic scattering by nuclei in the range of mass numbers between 64 and 238 has been studied with monochromatic photons in the energy range between 2 and 8 MeV. These photons were provided either by a Ti(n,γ) source installed in the tangential through channel of the Grenoble high flux reactor, or by24Na and56Co sources produced by deuteron bombardment of Al or Fe at the Göttingen cyclotron. The photoexcitation of 23 nuclear levels has been observed and the decay properties and groundstate widths of the majority of these levels have been determined. For the lead scattering target the coherent elastic differential cross section has been studied in detail. There is evidence that below the photo-neutron threshold the elastic scattering via virtual photoexcitation of the nucleus can be approximated by extrapolating the real part of the Giant Dipole Resonance amplitude along a Lorentzian curve. Coulomb corrections to Delbrück scattering seem to play a small role at 6.5 MeV.

Differential cross sections for the elastic scattering of γ-rays for energies from 145 keV To 750 keV

Abstract Differential cross sections for the elastic scattering of γ-rays for energies between 145 keV and 750 keV and for charge numbers from 30 to 82 are presented for scattering angles ranging from 45° to 135°. These differential cross sections have been calculated using form factors and “corrected form factors” obtained from relativistic HFS wave functions and using a semi- empirical procedure for obtaining accurate scattering amplitudes from the form factors. For an easy extension of the tabulation, exact analytical expressions based on relativistic hydrogen wave functions are presented for both form factors and all subshells of the atom. Effective charge numbers are given providing close agreement between form factors calculated from relativistic hydrogen wave functions and from relativistic HFS wave functions.

Elastic scattering of γ-rays from lead for energies from 0.145 to 1.33 MeV

Abstract Differential cross sections for elastic scattering of 889 and 1120 keV γ-rays from lead have been measured at angles ranging from 30° to 150°. These results and previously measured differential cross sections at 145, 279, 412, 662 and 1332 keV are compared with theoretical predictions taking into account Rayleigh scattering, nuclear Thomson scattering, and Delbruck scattering. The Rayleigh amplitudes of the K-shell were obtained from the theory of Brown et al., the amplitudes of the other shells from form factors calculated from relativistic HFS wave functions. Discrepancies are found at 145, 889, 1120 and 1332 keV while the experimental data at 279, 412 and 662 keV are in good agreement with the theory. A discussion of possible explanations and consequences is presented.

Isomerenverhältnisse für die Reaktionen107Ag(α,2n)109g,mIn und109Ag(α,2n)111g,mIn

Isomeric cross-section ratios have been measured for the reactions107Ag(α,2n)109g,mIn and109Ag(α, 2n)111g,mIn for energies of theα-particles between 19 and 40 MeV. The experimental isomeric cross-section ratios are compared with statistical-model calculations. The program ofHafner et al. was extended by including the competition of the (α,n)- and (α, 3n)-reactions and of charged particle emission. Agreement between experiment and theory could be achieved.

The energy dependence of Delbrück scattering investigated at Z = 73, 82 and 92☆

Abstract Using neutron capture γ-rays from a 140 CeO 2 source installed in the Grenoble high-flux reactor, differential cross sections for the elastic scattering of photons by Ta, Pb and U through θ = 120° have been measured for E = 4.291 and 4.767 MeV. These data have been supplemented by measuring elastic differential cross sections for U , θ = 120° and energies ranging from 0.279 to 1.332 MeV, using radioactive sources. The experimental differential cross sections below 1 MeV confirm the predicted Rayleigh amplitudes based on the second-order S -matrix within 3%. An excellent agreement between experiment and lowest-order Delbruck theory is observed between 1.0 and 1.4 MeV, showing that Coulomb corrections are small close to the threshold for pair production. At 4.291 and 4.767 MeV experiment and lowest-order Delbruck theory agree within ~12%

Delbrück scattering of 2.75 MeV photons by lead

Abstract The differential cross section for the elastic scattering of photons was measured for Pb at θ = 120° and E = 2.75 MeV yielding d σ/ d Ω = 34.7 ± 1.6 μ b / sr in agreement with d σ/ d Ω = 32.2 μ b / sr calculated using the Delbruck amplitudes of Papatzacos and Mork. Definite evidence for the real part of the Delbruck amplitude has been obtained for the first time.